This disclosure pertains to microlithography, which involves the transfer of an image of a pattern to the surface of a xe2x80x9csensitizedxe2x80x9d substrate using an energy beam such as ultraviolet light or a beam of charged particles. Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, thin-film magnetic-pickup heads, and micromachines. More specifically, the disclosure pertains to alignment marks that are used mainly for aligning and detecting the position of the substrate while performing a microlithographic exposure of the substrate. The disclosure also pertains to alignment-mark patterns, as defined on a reticle, corresponding to such alignment marks.
In recent years, the inability of optical microlithography to resolve increasingly finer pattern features has been an obstacle to obtaining currently desired levels of integration and miniaturization of microelectronic devices. Hence, large efforts are ongoing to develop a practical xe2x80x9cnext-generationxe2x80x9d microlithography technology that can achieve satisfactory resolution of substantially finer pattern features than resolvable using optical microlithography.
Microlithography performed using a charged particle beam is a major candidate next-generation microlithography technology. Charged-particle-beam (CPB) microlithography offers prospects of substantially better resolution than optical microlithography for reasons similar to the reasons for which electron microscopy yields better imaging resolution than optical microscopy. An ongoing technical challenge with current CPB microlithography approaches is the attainment of satisfactory throughput.
Another technical challenge with CPB microlithography is a more general problem that arises in any of various efforts to achieve substantially greater imaging resolution (i.e., resolution of smaller pattern linewidths), either by CPB microlithography or other type of microlithography. Namely, the greater the desired imaging resolution, the greater the required accuracy of position detection of, inter alia, the lithographic substrate.
Conventionally, detection of substrate position in a microlithography apparatus is accomplished using an alignment device that detects the respective positions of alignment marks associated with the substrate, e.g., alignment marks defined in or on the surface of the substrate. Exemplary optical-alignment devices include devices based on field image alignment (FIA), which detect alignment marks using a two-dimensional image sensor such as a CCD or the like and perform image processing to obtain position data. The alignment marks for FIA usually are situated on the substrate and are formed by exposing an alignment-mark pattern, defined by a mask or reticle, onto the surface of the substrate.
In view of the fact that many layers are formed on the substrate during fabrication of microelectronic devices on the substrate, alignment marks normally are formed at least in the first layer exposed onto the substrate. Hence, the reticle defining the circuit pattern to be formed in the first layer typically includes an alignment-mark pattern, and both the first-layer circuit pattern and the alignment marks are transferred lithographically from the reticle to the substrate. In addition to the first layer, one or more layers formed subsequent to the first layer also can include respective alignment marks that are transferred to the substrate as required.
Depending upon the type of microelectronic devices being fabricated on the substrate, lithographic pattern exposures can be performed using several types of microlithography apparatus rather than only one type. For example, some layers can be exposed using a CPB microlithography apparatus, and other layers can be exposed using an optical (deep ultraviolet) microlithography apparatus. The reason for this flexibility is that certain layers may have smaller minimum linewidths than other layers, and it is desirable from the standpoint of throughput and other concerns to utilize, for a particular layer, the most efficient lithographic exposure method that also produces the desired linewidth resolution. Hence, it is desirable that the alignment marks formed on the substrate be usable for obtaining high-accuracy position detection by optical means as well as by CPB means.
FIGS. 10(A)-10(B) depict respective examples of alignment marks usable for FIA (which, as noted above, is an optically based alignment-detection method and is used in conventional optical microlithography apparatus). The alignment marks consist of alignment-mark elements (denoted by respective shaded portions in the figures) defined as corresponding through-holes (apertures) in the respective mask. Regarding the alignment mark 10 shown in FIG. 10(A), certain elements 10V are oriented vertically in the figure and other elements 10H are oriented horizontally in the figure. The alignment-mark elements 10V, 10H intersect each other. Regarding the alignment mark 11 shown in FIG. 10(B), certain alignment-mark elements 11V are oriented vertically and other alignment-mark elements 11H are oriented horizontally. Respective groups of horizontal elements 11H and vertical elements 11V are arranged in respective parallel arrays, but do not intersect each other. The exemplary marks shown in FIGS. 10(A) and 10(B) provide good two-dimensional detection accuracy using optical alignment detectors and hence are used in many optical microlithography systems.
As noted above, different layers formed on a substrate need not be formed using the same microlithography technology. For example, one layer can be formed using CPB microlithography, and the next layer can be formed using optical microlithography. In the case of the marks shown in FIGS. 10(A)-10(B), if the first layer is to be formed on the substrate using CPB microlithography (wherein the respective pattern as well as the alignment-mark patterns are defined on a stencil reticle), a problem arises with respect to the alignment-mark patterns as defined on the reticle. Specifically, in the case of the alignment mark 10 shown in FIG. 10(A), the corresponding alignment-mark pattern as defined on the reticle is a xe2x80x9cdonutxe2x80x9d pattern. I.e., the alignment-mark elements 10V, 10H are defined by respective apertures in the reticle. However, as can be seen, the apertures completely surround xe2x80x9cislandsxe2x80x9d 10I in a donut manner, leaving the islands 10I without any physical support on the reticle. Complete xe2x80x9cdonutxe2x80x9d elements cannot be defined on a stencil-type reticle. In the case of the alignment mark 11 shown in FIG. 10(B), defining the corresponding alignment-mark pattern 11 on a stencil reticle poses a high probability of deformation of the alignment-mark elements 11H, 11V (e.g., warping and/or twisting) due to stress in the reticle.
In view of the foregoing, if exposure of the first layer is to be performed by CPB microlithography using a stencil reticle, then there is an urgent need for alignment marks that are definable in the first layer and that can be detected with good accuracy and precision using an optically based alignment-detection device.
To address the above and other shortcomings of conventional alignment marks and associated methods, the present invention provides, inter alia, alignment-mark patterns that can be defined on a stencil reticle for transfer using a charged particle beam to a sensitized substrate such as a resist-coated semiconductor wafer. As imprinted on the substrate, the corresponding alignment marks can be used for highly accurate alignments performed using an optically based alignment-detection device (e.g., a detection device based on FIA).
According to a first aspect of the invention, alignment-mark patterns are provided that are defined on a stencil reticle used in charged-particle-beam microlithography. The alignment-mark patterns are configured to be lithographically transferred by a charged particle beam from the stencil reticle to a sensitized substrate so as to imprint on the substrate a corresponding alignment mark detectable using an optical-based alignment-detection device. An embodiment of such an alignment-mark pattern comprises pattern elements defined as respective apertures in the stencil reticle. Each of the pattern elements on the reticle is split into respective pattern-element portions that are separated by respective girders formed from a membrane of the stencil reticle. Splitting of the pattern elements and interposing girders between adjacent pattern-element portions avoid forming membrane islands in the reticle and prevent stress-based deformation of the pattern elements in the reticle. Such an alignment-mark pattern, when projected onto the surface of a suitable substrate, yields a corresponding alignment mark allowing substrate-position (alignment) detection to be performed with high accuracy in either a charged-particle-beam (CPB) microlithography apparatus or an optical microlithography apparatus. In other words, as various lithographic procedures are performed on the substrate, the same alignment marks can be used without sacrificing accuracy.
The alignment-mark pattern can comprise pattern elements that include intersecting pattern elements. In such an instance, the girders can extend across respective pattern elements at regions of intersection of the corresponding alignment-mark elements. The pattern elements also can include girders that extend across respective pattern elements at regions displaced from regions of intersection.
Alternatively, the alignment-mark pattern can be only of pattern elements that do not intersect with each other. In such an instance, the pattern elements can include a first group of pattern elements that are separate from but oriented perpendicularly to a second group of pattern elements.
Desirably, in the corresponding alignment mark on the substrate, each alignment-mark girder has a width that is no greater than a resolution limit of the optical-based alignment-detection device. Thus, alignment-measurement accuracy is not compromised compared to that obtained with an alignment mark transferred from an alignment-mark pattern that is not split.
According to another aspect of the invention, stencil reticles are provided for use in CPB microlithography. An embodiment of such a reticle comprises a reticle membrane and an alignment-mark pattern. The alignment-mark pattern comprises multiple pattern elements defined as respective apertures in the reticle membrane. The alignment-mark pattern is configured to be transferred lithographically by a charged particle beam from the stencil reticle to a sensitized substrate so as to imprint on the substrate a corresponding alignment mark that is detectable using an optical-based alignment-detection device. Each of the pattern elements on the reticle is split into respective pattern-element portions, separated by respective girders formed from the reticle membrane, so as to avoid forming membrane islands in the reticle and to prevent stress-based deformation of the pattern elements in the reticle. Generally, the reticle also defines a device pattern normally defined in a first region of the reticle, wherein the alignment-mark pattern is defined in a second region of the reticle separate from the first region.
In the various embodiments of a stencil reticle, the pattern elements of the alignment-mark patterns can have any of the various configurations summarized above.
The reticle membrane desirably is made of a charged-particle-scattering material, thereby avoiding absorption of incident charged particles by the reticle. Avoiding absorption reduces temperature increases otherwise experienced by reticles during use in a CPB microlithography apparatus. Reducing temperature increases results in suppression of pattern deformation as defined on the reticle.
According to another aspect of the invention, stencil reticles are provided that include an alignment-mark pattern such as any of those summarized above. The reticles desirably are scattering-stencil reticles.
Yet another aspect of the invention is directed to methods, in the context of microlithographic methods, for determining an alignment of a lithographic substrate. According to an embodiment, on a stencil reticle defining an alignment-mark pattern comprising pattern elements defined as respective apertures in a membrane of the stencil reticle, each of the pattern elements on the reticle is split into respective pattern-element portions. The pattern-element portions are separated from each other by respective girders formed from the membrane. This splitting of pattern elements with intervening girders avoids forming membrane islands in the reticle and prevents stress-based deformation of the pattern elements in the reticle. The alignment-mark pattern on the reticle is transferred lithographically to a sensitized substrate using a charged particle beam so as to imprint the corresponding alignment mark on the substrate. The alignment mark is detected to determine alignment of the substrate. Desirably, the detecting step is performed using an optical-based alignment-detection device, such as an FIA-based device.
In the defining step of the foregoing method embodiment, the alignment-mark elements can be defined in any of various ways as summarized above. The detecting step can be performed using an optical-based alignment-detection device. If so, then each alignment-mark girder desirably is configured to have a width that is no greater than a resolution limit of the optical-based alignment-detection device.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.